Coat a 12-cup Bundt pan with cooking spray; dust with breadcrumbs.
Lightly spoon flour into dry measuring cups; level with a knife.Combine flour, baking soda and salt, stirring well with a whisk.
Place sugar and butter in a large bowl; beat with a mixer at medium speed until light and fluffy (about 3 minutes).Add rind, juice and extract, beating until well blended.
Add egg whites and egg, 1 at a time, beating well after each addition.
Add flour mixture to sugar mixture alternately with sour cream, beginning and ending with flour mixture.Stir in walnuts.
Spoon batter into prepared pan.Bake at 350* for 50 minutes or until a wooden pick inserted in center comes out clean.Cool in pan 15 minutes on a wire rack; remove from pan.
Cool completely on wire rack.Sprinkle with powdered sugar.
